1. Field of the Invention
The present invention relates to a spectacle lens that is designed to reduce chromatic aberration.
2. Description of the Related Art
A spectacle lens is generally formed as a single lens element, so that chromatic aberration as an optical lens defect that is caused by the spectacle lens cannot be appropriately corrected to a negligible degree. In spectacle lenses, the occurrence of transverse chromatic aberration is a fundamental problem. Due to the occurrence of transverse chromatic aberration, the wearer of the spectacles can see color fringes when he or she sees things through peripheral portions of the lenses. Such a problem occurs since light rays coming in the same direction enter each eye of the wearer as if the light rays were coming in different directions depending on the differences in wavelength (red R, green G and blue B) as shown in FIG. 32. In FIG. 32 L and E represent a spectacle lens and an eye ball, respectively. When a single lens such as a spectacle lens is designed such that chromatic aberration is reduced without the use of any additional lens, using a lens material whose dispersive power is as small as possible (i.e., whose Abbe number is as large as possible) was formerly the only way to reduce chromatic aberration.
A crown glass or an optical plastic CR-39, whose Abbe number is approximately 60, is conventionally used as a material for making spectacle lenses. A glass material whose Abbe number is 60 can be said to have a small dispersive power. However, in the case where such a glass material is used for making a spectacle lens having a large diopter, color fringes seen through a peripheral portion of the lens becomes very apparent, and can become distracting.
In FIG. 33, the broken line shows chromatic aberration (chromatic aberration of magnification) in the case of viewing through a conventional specific spectacle lens (first prior art) in a direction with a visual angle of 50 degrees. This conventional spectacle lens is made of CR-39 (Abbe number: 60); refractive power: 1.50) and has a vertex power of -8.00 (diopter). The horizontal axis represents wavelength .lambda. (nm) while the vertical axis represents the angle of deviation (d.theta.) from the incident angle (.theta.) of a reference wavelength (G).
In recent years various optical plastic materials having a high refractive index have been produced for the purpose of making spectacle lenses thin and light-weight. However, in the case where an optical plastic material having a high refractive index is used for making a lens, the dispersive power of the lens tends to be large, i.e., the Abbe number of the lens tends to be small. Therefore, in order to reduce chromatic aberration it is not preferable that an optical plastic material having a high refractive index be used for making spectacle lenses.
In FIG. 33, the solid line shows chromatic aberration (chromatic aberration of magnification) in the case of viewing through another conventional spectacle lens (second prior art) in a direction with a visual angle of 50 degrees. This conventional spectacle lens is made of a polyurethane plastic material having a high refractive index (Abbe number: 32; refractive power: 1.66) and has a vertex power of -8.00 (diopter).
In FIG. 34, the broken line shows chromatic aberration (chromatic aberration of magnification) in the case of viewing through yet another conventional spectacle lens (third prior art) in a direction with a visual angle of 50 degrees. This conventional spectacle lens is made of CR-39 (Abbe number: 60; refractive power: 1.50) and has a vertex power of +4.00 (diopter). The horizontal axis represents wavelength .lambda. (nm) while the vertical axis represents the angle of deviation (d.theta.) from the incident angle (.theta.) of a reference wavelength (G).
In FIG. 34, the solid line shows chromatic aberration (chromatic aberration of magnification) in the case of viewing through yet another conventional spectacle lens (fourth prior art) in a direction with a visual angle of 50 degrees. This conventional spectacle lens is made of a polyurethane plastic material having a high refractive index (Abbe number: 32; refractive power: 1.6) and has a vertex power of +4.00 (diopter).
Japanese laid-open patent publication No. 7-28002 has disclosed a technique for reducing chromatic aberration such as mentioned above. According to the technique, a spectacle lens is made of a composite lens consisting of a plurality of lenses having different Abbe numbers. However, such a composite lens is thick and heavy, which is undesirable for spectacles.
Japanese laid-open patent publication No. 60-203913 has disclosed another technique utilizing the effect of diffraction for reducing chromatic aberration caused by an overall optical system including a spectacle lens and a corresponding eye of the user. However, the technique is referred to only longitudinal chromatic aberration, whereas transverse chromatic aberration, a fundamental problem to be overcome in spectacle lenses as mentioned above, is not dealt with.
Japanese laid-open patent publication No. 7-49471 and some other publications have disclosed various techniques utilizing effect of diffraction for making a multifocal spectacle lens, a multifocal contact lens or a multifocal intraocular lens. However, the refractive power of a lens due to diffraction depends largely on the wavelength of light ray passing through the lens, and also the degree of the dependency varies according to the variation in the order of diffraction. Therefore, there is a drawback in that the chromatic aberration (chromatic aberration of magnification) increases as the order of diffraction of the lens becomes large. Longitudinal chromatic aberration is a fundamental problem in the case of contact lenses and intraocular lenses while transverse chromatic aberration is a fundamental problem in the case of spectacle lenses. However, any method or technique for reducing chromatic aberration is not taught in any of the aforementioned publications at all, including Japanese laid-open patent publication No. 7-49471.
Japanese laid-open patent publication No. 64-50012 has disclosed a technique for making the front surface of a lens to be a rotationally-symmetrical aspherical surface so as to reduce the weight and thickness of the lens. The aforementioned spectacle lens as the second prior art is provided on a front surface thereof with such a rotationally-symmetrical aspherical surface. Table 1 below shows the specification of the spectacle lens.
TABLE 1 ______________________________________ Vertex power: SPH -8.00 (diopter) Paraxial radius of curvature of front surface: R1 620.336 (mm) Aspherical factor of front surface: K 0.000 A4 2.299 .times. 10.sup.-7 A6 -1.594 .times. 10.sup.-10 A8 6.101 .times. 10.sup.-14 A10 -1.210 .times. 10.sup.-17 Radius of curvature of rear surface: R2 73.223 (mm) Optical center thickness: tc 1.100 (mm) Refractive index: n 1.660 Diameter: .o slashed. 75.000 (mm) Rim thickness: te 9.975 (mm) ______________________________________
wherein the shape of a rotationally-symmetrical aspherical surface is defined by the following equation: EQU X=Ch.sup.2 /{1+[1-(1+K)C.sup.2 h.sup.2 ].sup.1/2 }+A4h.sup.4 +A6h.sup.6 +A8h.sup.8 +A10h.sup.10 +
wherein:
h designates a distance from the optical axis;
X designates a distance from a tangent plane of an aspherical vertex;
C designates a curvature of the aspherical vertex (1/r),
K designates a conic constant;
A4 designates a fourth-order aspherical factor;
A6 designates a sixth-order aspherical factor;
A8 designates a eighth-order aspherical factor; and
A10 designates a tenth-order aspherical factor.
The rim thickness (=9.975 mm) of this spectacle lens of Table 1 is smaller than the rim thickness (=10.734 mm) of a regular spherical lens having no aspherical surface (R1=305.720, R2=64.845). However, the rim thickness of this spectacle lens of Table 1 cannot be said to be sufficiently small.
The aforementioned spectacle lens as the fourth prior art is provided on a front surface thereof with a rotationally-symmetrical aspherical surface such as mentioned above. Table 2 below shows the specification of the spectacle lens.
TABLE 2 ______________________________________ Vertex power: SPH +4.00 (diopter) Paraxial radius of curvature of front surface: R1 139.395 (mm) aspherical factor of front surface: K 0.000 A4 -5.518 .times. 10.sup.-7 A6 1.521 .times. 10.sup.-10 A8 -3.719 .times. 10.sup.-14 A10 5.176 .times. 10.sup.-18 Radius of curvature of rear surface: R2 795.455 (mm) Optical center thickness: tc 4.700 (mm) Refractive index: n 1.660 Diameter: .o slashed. 75.000 (mm) Rim thickness: te 1.231 (mm) ______________________________________
The optical center thickness (=4.700 mm) of this spectacle lens of Table 2 is smaller than the optical center thickness (=5.823 mm) of a regular spherical lens having no aspherical surface (R1=70.000, R2=114.761, te=1.231, .theta.=75). However, the optical center thickness of this spectacle lens of Table 2 cannot be said to be sufficiently small.